The invention relates to a control circuit which serves for security-critical driving of an electric consumer with an inductive load portion (e.g. direct current motor) to be connected to a direct voltage source.
Direct current motors are often controlled today with the aid of a power driving assembly integrated in the control circuit. The power driving assembly disconnects or connects the direct current motor electrically from/to the direct voltage source. A circuit construction suitable for this and known from the applicant's operational practice provides switching the power driving assembly as connecting link between a feed voltage source and the direct current motor. FIG. 1 illustrates an arrangement of the power driving assembly of this kind.
Citation (1) (DE 100 50 287) describes a protective device for the drive for unipolar direct current motors, which enables compact design of the drive circuit components and prevents thermal overload of the components.
A direct current drive device is known from citation (2) (DE 101 18 401, and corresponding U.S. Pat. No. 6,512,346 B2, both of which are incorporated by reference herein). The direct current drive device has a switching device on a first current path between a direct current supply and a direct current motor and a detection device which detects a voltage on a second current path between the direct current motor and the switching device.
The switching device has various switching elements and the detection device has various detection devices. Each switching element is provided on the first current path. Each detection element detects the voltage on the second current path. Each of the first current paths contains one of the second current paths in each case. An assessment device judges that there is a failure on a third current path from the direct current supply to the switching devices via the direct current motors if one of the voltages of the detection device does not change.
The control circuit 2 illustrated in FIG. 1 has, apart from the power driving assembly 4, a free-wheeling diode 6. The power driving assembly 4 has a power MOSFET (8) with three terminals (drain 14, source 16, gate 18). The drain terminal 14 is connected to the feed voltage source 20. A control assembly 22 serves to drive the MOSFET 8. It contains a charge pump (24), which delivers the gate voltage (18) of the MOSFET 8, and a microcontroller 26 to drive the MOSFET 8. The control assembly 22 is connected to the gate terminal 18 of the MOSFET 8 and via two further terminals returned to differently designed earths (GND_P 28 and GND 30). An ohmic resistor 66 is switched parallel to the gate source path of MOSFET 8 of the power driving assembly 4. The series circuit consisting of two breakdown diodes 68, 70 is likewise switched parallel to the gate source path of MOSFET 8 of the power driving assembly. Since the anodes of the breakdown diodes 68, 70 are switched together, their effect is comparable to that of an electric resistor or a bi-directionally operating limiting diode. The MOSFET 8 serves as semi-conductor switch, the respective switching status of which is fixed by the microcontroller 26. When the direct current motor 32 is electrically disconnected from the feed voltage source 20 by the power driving assembly 4, the inductive load portion of the direct current motor 32 generates an undesired voltage peak, among other things, on the basis of self-induction. The energy stored in the motor inductance and also energy from the feed voltage source 20 are in this case reduced via MOSFET 8 of the power driving assembly 4. To protect MOSFET 8 of the power driving assembly 4 a power diode, acting as free-wheeling diode 6, is switched parallel to the direct current motor 32. This is switched in the blocking direction in respect of the feed voltage source 20 and has the task of reducing the voltage peak occurring when the feed voltage source 20 is electrically separated from the direct current motor 32. The parallel switching consisting of direct current motor 32 and free-wheeling diode 6 is connected to the source terminal 16 of MOSFET 8. The second terminal of this parallel circuit is returned to earth 28.
A further development of the control circuit 2 illustrated in FIG. 1 provides for expanding the control circuit 2 by a reverse-connection protected MOSFET 12. At the same time the free-wheeling diode 6 from FIG. 1 is replaced by a free-wheeling MOSFET 10, the intrinsic diode 62 of which acts as a free-wheeling diode in blocking operation of the free-wheeling MOSFET 10. The resulting control circuit 34 with power driving assembly 4, free-wheeling MOSFET 10 and reverse-connection protected MOSFET 12 is illustrated in FIG. 2, as known from DE 10050287 A1.
The control circuit 34 illustrated in FIG. 2 has a power driving assembly 4 equivalent to the control circuit 2 in FIG. 1. The drain terminal 14 of MOSFET 8 of the power driving assembly 4 is connected to the feed voltage source 20. Driving of MOSFET 8 is done via the gate terminal 18 by a PCU 36 power control unit. A further terminal of the PCU 36 is returned to earth 28. The source terminal 16 of MOSFET 8 is connected to the direct current motor 32, a zero-point comparator 38 and the drain terminal 14 of the free-wheeling MOSFET 10. The direct current motor 32 and the zero-point comparator 38 are returned to earth 28 with a second terminal. To drive the free-wheeling MOSFET 1.0 its gate terminal 18 is connected to the zero-point comparator 38. The source terminal 16 of the free-wheeling MOSFET 10 and that of the reverse-connection protected MOSFET 12 form a direct connection. Drain 14 of the reverse-connection protected MOSFET 12 is returned to earth 28. To drive the reverse-connection protected MOSFET 12 its gate terminal 18 is connected to the feed voltage source 20 via an ohmic resistor 84. The zero-point comparator 38 is likewise connected to the feed voltage source 20. MOSFETS 8, 10, 12 of the control circuit 34 have in each case a series circuit, switched parallel to the gate source path, consisting of two breakdown diodes. Since the anodes of the two breakdown diodes of each of the three pairs of breakdown diodes 72, 74; 76, 78 and 80, 82 are switched together, the effect of each pair of breakdown diodes is comparable to that of an electric resistor or a bi-directionally operating limiting diode. MOSFETS 8, 10, 12 of the control circuit 34 further behave in blocking operation like a diode switched parallel to the MOSFET (intrinsic diode), the cathode of which is led through at the drain terminal 14 and the anode of which is led through at the source-terminal 16 of the MOSFET.
The mode of operation of the control circuit 34 illustrated in FIG. 2 is examined below. MOSFET 8 of the power driving assembly 4 here acts as semi-conductor switch. Controlled by a PCU 36, it connects the direct current motor 32 to the feed voltage source 20.
Functioning of the free-wheeling MOSFET 10 is controlled in the switched-off phases by the zero-point comparator 38. This identifies an electrical disconnection between the feed voltage source 20 and the direct current motor 32 with the aid of the negative potential at the source terminal 16 of MOSFET 8 of the power driving assembly 4. As a result of this the zero-point comparator 38 feeds the free-wheeling MOSFET 10 with a gate voltage. The free-wheeling MOSFET 10 now remains switched on during the entire switching off process. Compared with the free-wheeling diode 6 in FIG. 1, a smaller drop in voltage occurs via the switched-on free-wheeling MOSFET 10. Besides the reduction in losses in the free-wheeling MOSFET 10, at the same time a tail current in MOSFET 8 of the power driving assembly 4 during the switching off process is avoided. At the end of the switching off process the potential at the source terminal 16 of MOSFET 8 of the power driving assembly 4 again approaches that of the earth 28. The zero-point comparator 38 identifies this status and reduces the gate potential of the free-wheeling MOSFET 10 until it is operating purely as a diode. This avoids an undesired braking effect of the direct current motor 32 because of a negative current influence.
The reverse-connection protected MOSFET 12 has the task of protecting the free-wheeling MOSFET 10 from overload in the event of reversed polarity. This is achieved by the blocking behaviour of the reverse-connection protected MOSFET 12, which causes disconnection of the free-wheeling MOSFET 10 from the voltage supply.
Driving of the power driving assembly by the PCU is frequently based on a specific clock rate. The power driving assembly here disconnects and connects the direct voltage source electrically from/to the control circuit within a clock period.
Reducing the losses during the switching process by using a free-wheeling diode or a free-wheeling MOSFET is of great significance at low-frequency clock rates. The presence and failure-free functional ability of a free-wheeling diode or a free-wheeling MOSFET are an important precondition for the operation of the control circuit in security-critical applications. These include, for example, the ABS (anti-lock brake system) or the FDR (driving dynamics regulation) in automobile technology. The above-described control circuits have no facility for failure control. As a result they have a high risk priority number (RPN) in a failure mode and effects analysis (FMEA).
The probability of failure occurring is currently dependent exclusively on the production options and the failures occurring in practical use. However, an improvement in the production options is often not possible on technological or economic grounds.